1 9 ObjectOriented Programming Inheritance 1992 2007 Pearson

2 OBJECTIVES In this chapter you will learn: § How inheritance promotes software reusability. § The notions of superclasses and subclasses. § To use keyword extends to create a class that inherits attributes and behaviors from another class. § To use access modifier protected to give subclass methods access to superclass members. § To access superclass members with super. § How constructors are used in inheritance hierarchies. § The methods of class Object, the direct or indirect superclass of all classes in Java. 1992 -2007 Pearson Education, Inc. All rights reserved.

3 9. 1 Introduction • Inheritance – Software reusability – Create new class from existing class • Absorb existing class’s data and behaviors • Enhance with new capabilities – Subclass extends superclass • Subclass – More specialized group of objects – Behaviors inherited from superclass • Can customize – Additional behaviors 1992 -2007 Pearson Education, Inc. All rights reserved.

4 9. 1 Introduction (Cont. ) • Class hierarchy – Direct superclass • Inherited explicitly (one level up hierarchy) – Indirect superclass • Inherited two or more levels up hierarchy – Single inheritance • Inherits from one superclass – Multiple inheritance • Inherits from multiple superclasses – Java does not support multiple inheritance 1992 -2007 Pearson Education, Inc. All rights reserved.

5 9. 2 Superclasses and subclasses • Superclasses and subclasses – Object of one class “is an” object of another class • Example: Rectangle is quadrilateral. – Class Rectangle inherits from class Quadrilateral – Quadrilateral: superclass – Rectangle: subclass – Superclass typically represents larger set of objects than subclasses • Example: – superclass: Vehicle • Cars, trucks, boats, bicycles, … – subclass: Car • Smaller, more-specific subset of vehicles 1992 -2007 Pearson Education, Inc. All rights reserved.

6 Fig. 9. 1 | Inheritance examples. 1992 -2007 Pearson Education, Inc. All rights

7 9. 2 Superclasses and subclasses (Cont. ) • Inheritance hierarchy – Inheritance relationships: tree-like hierarchy structure – Each class becomes • superclass – Supply members to other classes OR • subclass – Inherit members from other classes 1992 -2007 Pearson Education, Inc. All rights reserved.

8 Fig. 9. 2 | Inheritance hierarchy for university Community. Members 1992 -2007 Pearson

9 Fig. 9. 3 | Inheritance hierarchy for Shapes. 1992 -2007 Pearson Education, Inc.

10 9. 3 protected Members • protected access – Intermediate level of protection between public and private – protected members accessible by • superclass members • subclass members • Class members in the same package – Subclass access to superclass member • Keyword super and a dot (. ) 1992 -2007 Pearson Education, Inc. All rights reserved.

9. 4 Relationship between Superclasses and Subclasses 11 • Superclass and subclass relationship – Example: Commission. Employee/Base. Plus. Commission. Employee inheritance hierarchy • Commission. Employee – First name, last name, SSN, commission rate, gross sale amount • Base. Plus. Commission. Employee – First name, last name, SSN, commission rate, gross sale amount – Base salary 1992 -2007 Pearson Education, Inc. All rights reserved.

9. 4. 1 Creating and Using a Commission. Employee Class 12 • Class Commission. Employee – Extends class Object • Keyword extends • Every class in Java extends an existing class – Except Object • Every class inherits Object’s methods • New class implicitly extends Object – If it does not extend another class 1992 -2007 Pearson Education, Inc. All rights reserved.

13 Software Engineering Observation 9. 3 The Java compiler sets the superclass of a class to Object when the class declaration does not explicitly extend a superclass. 1992 -2007 Pearson Education, Inc. All rights reserved.

Declare private instance variables Outline 14 Class Commission. Employee extends class Object Commission. Employee. java Implicit call to Object constructor Initialize instance variables (1 of 4) Line 4 Lines 6 -10 Line 16 Invoke methods set. Gross. Sales and set. Commission. Rate to Lines validate data 17 -21 Lines 20 -21 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 15 Commission. Employee. java (2 of 4) 1992 -2007 Pearson Education, Inc. All

Outline 16 Commission. Employee. java (3 of 4) Lines 85 -88 Calculate earnings 1992

Outline 17 Override method to. String of class Object Commission. Employee. java (4 of

Outline 18 Instantiate Commission. Employee object Commission. Employee Test. java (1 of 2) Lines 9 -10 Lines 15 -25 Use Commission. Employee’s get methods Line 26 -27 to retrieve the object’s instance variable values Use Commission. Employee’s set methods to change the object’s instance variable values 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 19 Implicitly call object’s to. String method Commission. Employee Test. java (2 of

20 9. 4. 2 Creating a Base. Plus. Commission. Employee Class without Using Inheritance • Class Base. Plus. Commission. Employee – Implicitly extends Object – Much of the code is similar to Commission. Employee • private instance variables • public methods • constructor – Additions • private instance variable base. Salary • Methods set. Base. Salary and get. Base. Salary 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 21 Base. Plus. Commission Employee. java Add instance variable base. Salary (1 of 4) Line 12 Line 24 Use method set. Base. Salary to validate data 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 22 Base. Plus. Commission Employee. java (2 of 4) 1992 -2007 Pearson Education,

Outline 23 Base. Plus. Commission Employee. java (3 of 4) 1992 -2007 Pearson Education,

Outline Method set. Base. Salary validates data and sets instance variable base. Salary 24 Base. Plus. Commission Employee. java (4 of 4) Lines 88 -91 Method get. Base. Salary returns the value of instance variable base. Salary Lines 94 -97 Line 102 Lines 108 -113 Update method earnings to calculate the earnings of a base-salaried commission employee Update method to. String to display base salary 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 25 Instantiate Base. Plus. Commission. Employee object. Base. Plus. Commission Employee. Test. java (1 of 2) Line 9 -11 Lines 16 -27 Use Base. Plu. Commission. Employee’s get methods to retrieve the object’s instance variable values 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 26 Use Base. Plus. Commission. Employee’s set. Base. Salary methods to set base salary Explicitly call object’s to. String method Base. Plus. Commission Employee. Test. java (2 of 2) Line 29 Line 33 Program output 1992 -2007 Pearson Education, Inc. All rights reserved.

27 9. 4. 3 Creating a Commission. Employee. Base. Plus. Commiion. Employee Inheritance Hierarchy • Class Base. Plus. Commission. Employee 2 – – – Extends class Commission. Employee Is a Commission. Employee Has instance variable base. Salary Inherits public and protected members Constructor not inherited 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 28 Base. Plus. Commission Employee 2. java Class Base. Plu. Commission. Employee 2 is a subclass of Commission. Employee (1 of 3) Line 4 Line 13 Invoke the superclass constructor using the superclass constructor call syntax 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 29 Base. Plus. Commission Compiler generates errors because superclass’s Employee 2. java instance variable commission. Rate and gross. Sales are private (2 of 3) Line 34 Lines 41 -46 Compiler generates errors because superclass’s instance variable first. Name, last. Name, social. Security. Number, gross. Sales and commission. Rate are private 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 30 Base. Plus. Commission Employee 2. java (3 of 3) Compiler generated errorss

9. 4. 4 Commission. Employee. Base. Plus. Commission. Employee Inheritance Hierarchy Using protected Instance Variables 31 • Use protected instance variables – Enable class Base. Plus. Commission. Employee to directly access superclass instance variables – Superclass’s protected members are inherited by all subclases of that superclass 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline Declare protected instance variables 32 Commission Employee 2. java (1 of 4) Line

Outline 33 Commission Employee 2. java (2 of 4) 1992 -2007 Pearson Education, Inc.

Outline 34 Commission Employee 2. java (3 of 4) 1992 -2007 Pearson Education, Inc.

Outline 35 Commission Employee 2. java (4 of 4) 1992 -2007 Pearson Education, Inc.

Outline 36 Base. Plus. Commission Employee 3. java Must call superclass’s (1 of 2)

Outline 37 Base. Plus. Commission Employee 3. java Directly access superclass’s protected (2 of

Outline 38 Base. Plus. Commission Employee. Test 3. java (1 of 2) 1992 -2007

Outline 39 Base. Plus. Commission Employee. Test 3. java (2 of 2) Program output

9. 4. 4 Commission. Employee. Base. Plus. Commission. Employee Inheritance Hierarchy Using protected Instance Variables (Cont. ) 40 • Using protected instance variables – Advantages • subclasses can modify values directly • Slight increase in performance – Avoid set/get method call overhead – Disadvantages • No validity checking – subclass can assign illegal value • Implementation dependent – subclass methods more likely dependent on superclass implementation – superclass implementation changes may result in subclass modifications • Fragile (brittle) software 1992 -2007 Pearson Education, Inc. All rights reserved.

9. 4. 5 Commission. Employee. Base. Plus. Commission. Employee Inheritance Hierarchy Uing private Instance Variables 41 • Reexamine hierarchy – Use the best software engineering practice • Declare instance variables as private • Provide public get and set methods • Use get method to obtain values of instance variables 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline Declare private instance variables 42 Commission Employee 3. java (1 of 4) Lines

Outline 43 Commission Employee 3. java (2 of 4) 1992 -2007 Pearson Education, Inc.

Outline 44 Commission Employee 3. java (3 of 4) 1992 -2007 Pearson Education, Inc.

Outline 45 Use get methods to obtain the Commission values of instance variables Employee

Outline 46 Base. Plus. Commission Employee 4. java Inherits from Commission. Employee 3 (1

Outline 47 Base. Plus. Commission Invoke an overridden superclass method from a. Employee 4. java subclass (2 of 2) Line 33 &the 40 Use get methods to obtain values of instance variables Line 33 Lines 40 Invoke an overridden superclass method from a subclass 1992 -2007 Pearson Education, Inc. All rights reserved.

48 Common Programming Error 9. 3 When a superclass method is overridden in a subclass, the subclass version often calls the superclass version to do a portion of the work. Failure to prefix the superclass method name with the keyword super and a dot (. ) separator when referencing the superclass’s method causes the subclass method to call itself, creating an error called infinite recursion. Recursion, used correctly, is a powerful capability discussed in Chapter 15, Recursion. 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 49 Base. Plus. Commission Employee. Test 4. java Base. Plus. Commission. Employee 4 Create object. (1 of 2) Lines 9 -11 Lines 16 -25 Use inherited get methods to access inherited private instance variables Use Base. Plus. Commission. Employee 4 get method to access private instance variable. 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 50 Use Base. Plus. Commission. Employee 4 set method to modify private instance variable base. Salary. Base. Plus. Commission Employee. Test 4. java (2 of 2) 1992 -2007 Pearson Education, Inc. All rights reserved.

51 9. 5 Constructors in Subclasses • Instantiating subclass object – Chain of constructor calls • subclass constructor invokes superclass constructor – Implicitly or explicitly • Base of inheritance hierarchy – Last constructor called in chain is Object’s constructor – Original subclass constructor’s body finishes executing last – Example: Commission. Employee 3 Base. Plus. Commission. Employee 4 hierarchy • Commission. Employee 3 constructor called second last (last is Object constructor) • Commission. Employee 3 constructor’s body finishes execution second (first is Object constructor’s body) 1992 -2007 Pearson Education, Inc. All rights reserved.

52 Software Engineering Observation 9. 8 When a program creates a subclass object, the subclass constructor immediately calls the superclass constructor (explicitly, via super, or implicitly). The superclass constructor’s body executes to initialize the superclass’s instance variables that are part of the subclass object, then the subclass constructor’s body executes to initialize the subclass-only instance variables. (cont…) 1992 -2007 Pearson Education, Inc. All rights reserved.

53 Software Engineering Observation 9. 8 Java ensures that even if a constructor does not assign a value to an instance variable, the variable is still initialized to its default value (e. g. , 0 for primitive numeric types, false for booleans, null for references). 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 54 Commission. Employee 4. java (1 of 4) Lines 23 -24 Constructor outputs

Outline 55 Commission. Employee 4. java (2 of 4) 1992 -2007 Pearson Education, Inc.

Outline 56 Commission. Employee 4. java (3 of 4) 1992 -2007 Pearson Education, Inc.

Outline 57 Commission. Employee 4. java (4 of 4) 1992 -2007 Pearson Education, Inc.

Outline 58 Base. Plus. Commission Employee 5. java (1 of 2) Constructor outputs message

Outline 59 Base. Plus. Commission Employee 5. java (2 of 2) 1992 -2007 Pearson

Outline 60 Instantiate Commission. Employee 4 object Constructor. Test. java (1 of 2) Lines 8 -9 Instantiate two Base. Plus. Commission. Employee 5 objects to demonstrate. Lines order 12 -19 of subclass and superclass constructor method calls. 1992 -2007 Pearson Education, Inc. All rights reserved.

Outline 61 Constructor. Test. java (2 of 2) Subclass Base. Plus. Commission. Employee 5 constructor body executes after superclass Commission. Employee 4’s constructor finishes execution. 1992 -2007 Pearson Education, Inc. All rights reserved.

62 9. 6 Software Engineering with Inheritance • Customizing existing software – Inherit from existing classes • Include additional members • Redefine superclass members • No direct access to superclass’s source code – Link to object code – Independent software vendors (ISVs) • Develop proprietary code for sale/license – Available in object-code format • Users derive new classes – Without accessing ISV proprietary source code 1992 -2007 Pearson Education, Inc. All rights reserved.